From my recollections of FMC/IRS alignment The following are true.
Leaving the switches in align will allow a more accurate alignment of the accelerometers.
Failure to insert ppos when align selected and more than 10 mins elapsed, will cause an error code in the IRS CDU (6 I think when STS is selected, but I may be wrong), a prompt on the FMC CDU scratchpad and the align lights on the IRS CDU will flash. This is because the ppos is req'd for full align, at this point the IRS will NOT further reduce errors in accelerometer alignment.
ARP is NOT req'd for alignment. Most operators I have worked with load gate ppos from the jepps, although some which only operate to a few airports have them loaded into the FMS database. Most use gate ppos to reduce map shift on T/O (unsquat) which on most a/c I have worked with is when update takes place. Failure to do this on airbus a/c may give rise to a IRS/FMS position error on the PFR. Of course the quick way to put in ppos is to use the last pos from the FMC (This is how most maint personnel do it.)
FMS uses nav info for position computation in the following order
GPS, DME/DME, DME/VOR, VOR,VOR, IRS.
Raw IRS position IS sometimes used. Our fleet has several a/c that are not GPS equiped but fly over the pond to the 'States. This can result in a large map shift when land is near and the first DME's are picked up (3+3T nm is the rule for IRS/INS error so after 4 hrs with no DME/VOR the IRS could be as much as 15nm out. In practice it is much less)
That's all my foggy memory can dredge up but hope it helps.